1. Field of the Invention
The present invention relates to measurement systems, designed to read the value of a parameter, such as pressure, flow control, strain, chemical properties, or temperature, where the sensor is connected to a measurement box, optionally with a length of cable or other components between the two, for the measurement box to give a value for the parameter being measured. The present invention relates to such systems where different sensors can be attached to different measurement boxes. The example given for the embodiment of the present invention particularly relates to, but is not limited to, systems for reading parameters where the measurement box can be separated from the transducer by a large length, perhaps some kilometres in length, of fibre optic cable, and where the transducer is an optical transducer.
2. Description of Related Art
High precision optical measurement instruments, for example for measuring pressure at the bottom of a well bore in a hydrocarbon well, usually comprise a transducer situated within the well bore, a fibre optic cable leading to the transducer, at the distal end of which the transducer is situated, and measurement equipment, out of the well bore, for interrogating the transducer through the fibre optic cable and providing output indicative of the pressure to which the transducer is instantly subject.
The individual transducers are generally individually calibrated, with their individual measurement instruments, by subjecting the individual transducer to controlled values of the parameter to be measured (e.g. pressure) and noting the value of the output of the measuring equipment. In general, all transducers, even those of the same type, give different outputs from each other for the same parametric stimulus, and all measuring equipment, even those of the same type, give different readings from each other for the same output from a transducer. This is due to differences in the manufacturing process, dimensions, material properties, and alignment, among others, from sensor to sensor or from box to box. The present invention seeks to provide a method and means for allowing different measuring equipment to be connected to different sensors without loss of accuracy and without having to re-calibrate the entire system.